The embodiments herein relate generally to subterranean formation operations and, more particularly, to drilling fluids for use in horizontal directional drilling.
Horizontal directional drilling (HDD) typically employs a steerable, trenchless method for forming wellbores to install underground utilities (e.g., underground pipes, conduits, cables, and the like). Conventional directional boring techniques traditionally operate from a boring device that pushes or steers a series of connected drill strings with a directable drill bit at the distal end thereof to achieve the wellbore geometry. A water-based drilling fluid containing finely divided bentonite clay particles and/or other additives is typically used in HDD operations. The drilling fluid is pumped down the drill string(s) through the openings in the drill bit and upwardly, through the annual space between the drill string(s) and the wall of the wellbore, to the surface of the earth. One of the primary benefits of employing the drilling fluid is to suspend the cuttings produced by the drill bit during the process of boring the wellbore and to transport these cuttings to the surface of the earth. Another important function of drilling fluid is to provide a thin and impermeable filter cake on the wall of the wellbore to reduce loss of water from the wellbore to the formation. Water infiltration from the drilling fluid to the surrounding formation may cause the formation to soften, which further induces wellbore sloughing and cave in. Other advantages of drilling fluid include lubricating the bit and the drill string, cooling the bit, reducing torque and drag, and providing a hydrostatic pressure head to prevent formation fluid from flowing into the wellbore.
Accordingly, gel strength and fluid loss are two important requirements of a drilling fluid for use in horizontal directional drilling As used herein, the term “gel strength” refers to the measurement of the drilling fluid's ability to support suspended cuttings when the fluid is at rest. A drilling fluid should have sufficient gel strength in order to keep cuttings from settling. If a drilling fluid does not exhibit an adequate gel strength, cuttings in the drilling fluid tend to settle out, which may result in sliding cuttings beds, stuck drill pipe, or total loss of the wellbore. Another requirement of a drilling fluid for boring any hole, either vertically or directionally, is a low fluid loss or filtration rate as referred to in the oil and gas well drilling industry.
A drilling fluid may have sufficient gel strength to prevent settling of cuttings when circulation of the drilling fluid is stopped. However, if the drilling fluid does not have a low filter loss, it may result in the deposition of a thick filter cake on the walls of the wellbore, which may cause instability of the wellbore. When large amounts of water filter from the drilling fluid into the formation surrounding the wellbore, the solids of the drilling fluid remain as a filter cake, thus reducing the size of the annular passage. Build-up of a thick filter cake may cause the stuck drill pipe, particularly in horizontal directional drilling operations. Moreover, stuck drill pipe is an increased risk in unconsolidated formations, such as silt or sandy soil.